Now to the latest provocative move from north korea. Days after declaring a state of war with south korea, the regime is restarting a nuclear reactor that could fuel their weapons program. Martha raddatz just returned from an exclusive tour of the north korean border with america's commanding general in the region. She joins us now. Good morning, martha. Reporter: Good morning, george. There have been so many threats but this threat to restart the nuclear reactor adds a new and frightening dimension. We stood just yards from north korean soldiers along a demilitarized zone watching our every move in what general james thurman described as a dangerous and volatile situation. When kim jong-un says threatens the united states with a nuclear attack you take that seriously. Well, I think we should and i think we need to make sure that we look at all our vulnerabilities. Reporter: But there is the more immediate threat as well. Stay ready. Reporter: What is your greatest fear right now with kim jong-un? A miscalculation and an impulsive decision that causes a kinetic provocation. There's 14,000 tubes of artillery just across this line beyond that far mountain range over there. They have a lot ofunderground facilities over there. They move this artillery around quite a bit and learned some lessons coming out of world war ii to not be unready. Reporter: Are you saying you'd respond with an overwhelming force? I'm saying that we will defend ourselves. Reporter: But he said he would respond carefully to provocation. What we don't want to do is respond to some type of deceptive move that would get us into a rapid escalation into a conflict and I think very hard about that because, again, my job is to prevent war. Reporter: But the trust on both sides has been badly shaken. The north korean soldiers watched us even inside taking pictures of this four-star general who said in his two years here he has never seen it so tense. I asked general thurman if he wanted to add any aircraft or ships. He said not at this time but, george, he is not taking anything off the table. And what are they worried about most that could spark a confrontation? Reporter: Well, I think what he's talking about is the artillery, just firing across that border or firing into seoul. He is really trying to tone down the situation, but at the very same time north korea is trying to ratchet it up.

This transcript has been automatically generated and may not be 100% accurate.

{"id":18860689,"title":"US General in North Korea: 'We'll Defend Ourselves'","duration":"2:34","description":"Martha Raddatz talks exclusively with General Thurman on the North Korean border.","section":"GMA","mediaType":"Default"}